libpanelapplet-2.0: Fix PanelApplet::change_background binding
libpanelapplet-2.0: Fix PanelApplet::change_background binding

Based on patch by Yu Feng, fixes bug 555566.

rvu95/vala,phako/vala,xdave/pvala,tjol/vala-tjol,GNOME/vala,severinh/vala,severinh/vala,themiwi/vala,frida/vala,frida/vala,severinh/vala,flobrosch/vala-fixes,kamanashisroy/aroop,playya/vala,playya/vala,themiwi/vala,frida/vala,GNOME/vala,Jc2k/vala,Distrotech/vala,flobrosch/vala-fixes,playya/vala,tjol/vala-tjol,Distrotech/vala,frida/vala,kamanashisroy/aroop,Distrotech/vala,themiwi/vala,jessevdk/vala,kamanashisroy/aroop,phako/vala,lovebug356/vala,spidermonkdust/vala,frida/vala,rvu95/vala,rvu95/vala,lovebug356/vala,spidermonkdust/vala,spidermonkdust/vala,Jc2k/vala,jessevdk/vala,flobrosch/vala-fixes,GNOME/vala,GNOME/vala,jessevdk/vala,tjol/vala-tjol